Among rare earth permanent magnets, R-T-B system sintered magnets have been used in various electric devices because the R-T-B system sintered magnets are excellent in magnetic properties, and Nd as the main component thereof is abundant as a source and relatively inexpensive. However, such R-T-B system sintered magnets with excellent magnetic properties also suffer from several technical problems to be solved. Among such problems is a fact that the R-T-B system sintered magnets are low in thermal stability, and hence undergo remarkable coercive force decrease brought about by temperature elevation. Accordingly, Patent Document 1 (Japanese Patent Publication No. 5-10806) has proposed that the coercive force at room temperature is enhanced by adding a heavy rare earth element typified by Dy, Tb or Ho, so as to enable the coercive force to be maintained to a level ensuring the use of the R-T-B system sintered magnets without trouble even when the coercive force is decreased by temperature elevation. The R2T14B compounds using these heavy rare earth elements are higher in anisotropic magnetic field than the R2T14B compounds using light rare earth elements such as Nd and Pr, and can attain a high coercive force.
An R-T-B system sintered magnet comprises a sintered body at least comprising main phase grains comprising an R2T14B compound and a grain boundary phase containing R in a larger content than the main phase. A proposal on the optimal concentration distribution of the heavy rare earth element in the main phase grains, having significant effects on the magnetic properties, and on the controlling method of the optimal concentration distribution is disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 7-122413) and Patent Document 3 (Japanese Patent Laid-Open No. 2000-188213).
Patent Document 2 has proposed that in a rare earth permanent magnet comprising, as the configuration phases thereof, a main phase mainly comprising the R2T14B grains (R represents one or more rare earth elements, and T represents one or more transition metals) and an R rich phase (R represents one or more rare earth elements), a heavy rare earth element is made to distribute so as to be high in concentration at least at three points in the R2T14B grains. The R-T-B system sintered magnet of Patent Document 2 is disclosed to be obtained as follows: an R-T-B system alloy comprising R2T14B as the configuration phase thereof and an R-T system alloy in which the area proportion of R-T eutectics containing at least one heavy rare earth element is 50% or less are pulverized separately and mixed together, and the mixture thus prepared is compacted and sintered to yield the R-T-B system sintered magnet. The R-T-B system alloy preferably comprises the R2T14B grains as the configuration phase thereof and is recommended to have a composition in which 27 wt %≦R≦30 wt %, 1.0 wt %≦B≦1.2 wt % and the balance is composed of T.
Additionally, Patent Document 3 discloses that an R-T-B system sintered magnet having microstructures containing first R2T14B type main phase grains higher in the concentration of a heavy rare earth element than the grain boundary phase and second R2T14B type main phase grains lower in the concentration of the heavy rare earth element than the grain boundary phase has a high residual magnetic flux density and a high value of the maximum energy product.
For the purpose of obtaining the above-described microstructures, Patent Document 3 adopts a so-called mixing method in which two or more R-T-B system alloy powders different in the content of the heavy rare earth element such as Dy are mixed together. In this case, the composition of each of the R-T-B system alloy powders is regulated in such a way that the total content of the R elements is the same in each of the alloy powders. For example, in the case of Nd+Dy, one of the alloy powders is set to have a composition of 29.0% Nd+1.0% Dy and the other of the alloy powders is set to have a composition of 15.0% Nd+15.0% Dy. Additionally, it is described that preferably the contents of the elements other than the R elements in the individual alloy powders are substantially the same.
Patent Document 1: Japanese Patent Publication No. 5-10806
Patent Document 2: Japanese Patent Laid-Open No. 7-122413
Patent Document 3: Japanese Patent Laid-Open No. 2000-188213